1. Field of the Invention
The present invention relates to a method of manufacturing spherical silica particles, and more particularly to a method of manufacturing spherical silica particles with high solidity and with adjusted surface smoothness, obtained through the sintering of spherical granulated silica obtained by spray-drying a fine particle silica suspension.
The spherical silica particles according to the present invention can be suitably used as a filler for resin composition (hereinafter referred to as sealant) for encapsulating IC electronic parts.
2. Prior Art
Recently, the chip area which contacts a sealant is increasing, and packages are becoming thinner as ICs are highly integrated. The demand in sealant quality is getting increasingly severe in order to protect chips with a thin sealant.
The first problem is heat stress resistance. Because heat stress is caused by the difference in the thermal expansion coefficient of both a silicon chip and sealant, adhesiveness of the sealant to the chip deteriorates. So, to bring thermal expansion coefficient of the sealant close to that of the silicon chip, a method is used where as large an amount of silica as possible, which has a low thermal expansion coefficient, is added to a resin as a filler.
Conventionally, crushed silica has been used for filler silica which is manufactured through crushing and has irregular shapes and sharp angles. Sealants with a high filling ratio of this crushed silica, however, had poor fluidity because of their high viscosity during molding. It thus has been become impossible to achieve homogeneous packages with the prescribed characteristics. Moreover, sharp-angled, crushed silica would wear down a mold and potentially lead to damage of the aluminum wiring on chips by piercing the protective sealant on the chip surface. Spherical silica with good fluidity and no sharp angles has therefore been ardently sought.
Among the conventional methods of manufacturing spherical silica particles which have been proposed are the following:
1) A flame-fusing method for crushed silica (for example, Japanese Patent Application Kokai No. 58-145613); PA1 2) A method for a sol solution obtained from a hydrolysis of an alkyl silicate, wherein the sol solution is sprayed into a heated medium, dryed, granulated and flame-fused (for example, Japanese Patent Application Kokai No. 58-2233); PA1 3) A method for partially condensed sol obtained from a hydrolysis of a silicon alkoxide, wherein the alcohol is removed from the sol, which is subsequently dispersed in water and the precipitated silica gel is baked (for example, Japanese Patent Application Kokai No. 63-225538).
The following problems arise in the conventional methods of manufacturing spherical silica particles described above.
The particles obtained from the Methods 1) and 2) have a small adhesiveness toward a resin because their surfaces are too smooth, making them inappropriate as a filler for sealants. The flame-fusing method, furthermore, imposes economic restrictions because of the quantities of expensive hydrogen and oxygen consumed. The high cost of the raw material in the Method 2) is further compounded by the need to treat the effluent which contains organic substances derived from the raw material.
The particles obtained from Method 3) are ideally shaped for a sealant filler, but the complexity of processes and operations involved, the expense of the raw materials, and the need for waste water treatment remain incumbent problems.
In order to prevent the emergence of soft errors attendant upon an enhancement of the degree of integration of IC electronic parts, highly pure filler has been sought for the employed sealant.
Replying to this demand are high-purity spherical silica particles with low content in such impurities as alkali metals, alkaline earth metals, halogens or radioactive substances.